The present invention relates generally to wireless communication and, more particularly, to a system and method for selectively protecting encoded information bits in M-ary quadrature amplitude modulation (QAM) transmissions.
As a function in the modulation and transmission process, the output of turbo encoder consists of systematic bits and punctured parity bits as shown in FIG. 1. A different code rate is realized by puncturing the parity bits at a certain rate.
In the prior art [1], and as shown in FIG. 2, the output of a turbo, or any other suitable encoder is interleaved without any consideration of importance of each code bit, or without consideration of the protection level afforded in the modulation of the encoded bits. In another prior art reference [2], the punctured parity bits are mapped to the most well-protected modulation bits without any justification.
In a third prior art reference [3], a turbo trellis modulation scheme is employed by jointly designing high order modulation and turbo code. FIG. 3 shows the turbo trellis modulation scheme where the modulation symbol boundary in time is kept by interleaving at the modulation symbol level rather than interleaving, and scattering in time the code bits. Here the systematic bits are mapped to the most well-protected modulation bits. However, this turbo trellis modulation scheme is not applicable to conventional wireless communication systems which require an interleaver between the channel encoder and the modulation symbol mapper. Since a fading channel often causes burst errors, wireless communications systems use a code bit interleaver to protect against burst errors which cause the decoder to lose both systematic and parity code bits for the same information bit during a fading period. Losing both systematic and parity code bits for a single information bit is worse for turbo decoder performance than losing a systematic bit of one information bit and a parity bit of another information bit. So, the output of the channel encoder should be interleaved before the encoder output is mapped to modulation symbols.
Thus, although it is well known that the mapping of information into modulation symbols provides different levels of noise immunity or protection, this principle has yet to be meaningfully applied to conventional wireless communication systems.
[1] TIA 1XTREME Ad hoc group, IS-2000 1XTREME Delta Specification, V2.0d
[2] S. Le Goff, A. Glavieux and C. Berrou, xe2x80x9cTurbo-codes and high spectral efficiency modulation,xe2x80x9d IEEE ICC 94 Proceedings, pp. 645-649, 1994.
[3] P. Robertson and T. Wortz, xe2x80x9cExtensions of turbo trellis coded modulation to high bandwidth efficiencies,xe2x80x9d IEEE ICC 97 Proceedings, pp. 1251-1255, 1997
The invention achieves better channel decoder performance than the prior art by providing a greater level of protection to the more important encoded bits, such as the systematic bits, with little additional hardware.
Modern communication systems provide higher rates of data transmission through the use of multiple level modulation such as M-ary quadrature amplitude modulation (QAM). Each bit in the QAM constellation is protected against channel impairment at a different level. On the other hand, each code bit of channel encoder output has different importance level in terms of decoding. So, a method and apparatus are described herein for efficiently mapping important code bits to well-protected input bits for modulation symbol mapper, and for a corresponding receiving method and apparatus. The present invention takes advantage of the inherent structure of such a priority conversion to provide improved decoder performance and less hardware complexity.
In most wireless communication systems the output of the channel encoder is typically interleaved before the encoder output is mapped to modulation symbols, since in wireless communication systems a fading channel often causes burst errors. The present invention method and apparatus provides a means of interleaving, while matching important code bits to well-protected modulation bits.